Head tube

A bicycle head tube with a partially installed bicycle headset; the locknut has yet to be fitted onto the fork steer tube.

The head tube is the part of a cycle's tubular frame within which the front fork steer tube is mounted.[1] On a motorcycle, the "head tube" is normally called the steering head. On bicycles the manufacturer's brand located on the head tube is known as a head badge.

The steering axis angle, also called caster angle, is the angle that the head tube and hence the steering axis makes with the horizontal or vertical, depending on convention. The steering axis is the axis about which the steering mechanism (fork, handlebars, front wheel, etc.) pivots.

In bicycles, the steering axis angle is called the head angle and is measured clock-wise from the horizontal when viewed from the right side. A 90° head angle would be vertical. For example, Lemond[5] offers:

a 2007 Filmore, designed for the track, with a head angle that varies from 72.5° to 74° depending on frame size

a 2006 Tete de Course, designed for road racing, with a head angle that varies from 71.25° to 74°, depending on frame size.

In motorcycles, the steering axis angle is called the rake and is measured counter-clock-wise from the vertical when viewed from the right side. A 0° rake would be vertical. For example, Moto Guzzi[6] offers:

The head tube of a bicycle is sometimes designated by the fork steerer column it accepts. This can lead to confusion, since head tube inside diameters are dependent on the headset standard. For example, frames that take one inch (25.4 mm) steerer columns can have three different inside diameters for threaded and threadless headsets (not including integrated-type headsets). The wide variety of integrated and non-standard, proprietary headsets that some frame manufacturers have created (and abandoned in some cases) makes listing all current and past head tube dimensions problematic. The following table includes the most common sizes; nominal head tube diameters are assuming a 0.1-0.2 mm interference fit, which is what most head tube reaming cutters are designed to bore. Adequate press fits are typically between 0.1 and 0.25 mm of interference.

Steerer column nominal OD

Headset standard

Head tube nominal ID

1"/25.4 mm

1" JIS

28.8 mm

1" ISO

30.0 mm

1" BMX/OPC

32.5 mm

11⁄8"/28.6 mm

11⁄8" standard, threaded and threadless

33.8 mm

IS 11⁄8" integrated 45×36

41.1 mm

Campagnolo "Hiddenset" 45×45

41.9 mm

Zero Stack/ Internal

43.9 mm

11⁄4"/31.75 mm

11⁄4" standard

36.8 mm

1.5"/ 38.1 mm

1.5" standard

49.6 mm

Motorcycles

Standard motorcycle head tubes and headsets are sized for a one inch (25.4 mm) diameter fork steerer tube.